1. Technical Field
Example embodiments relate generally to semiconductor memory devices, and to methods of operating nonvolatile memory devices.
2. Description of the Related Art
As the operating speed of a semiconductor memory device has increased, swing widths of signals interfaced between the semiconductor memory device and a memory controller have decreased. A reason for the decrease of the swing widths is to minimize the time required to transmit the signals. However, as the swing widths have decreased, the signals transferred between the semiconductor memory device and the memory controller may be more easily distorted by impedance mismatches caused by process, voltage and temperature (PVT) variations. An impedance calibration operation for adjusting an output impedance and/or a termination impedance of the semiconductor memory device can be employed at transmitting and/or receiving stages of the semiconductor memory device. In the impedance calibration operation, the output impedance and/or the termination impedance may be adjusted by comparing the output impedance and/or the termination impedance with an impedance of an external resistor. The impedance calibration operation may be referred to as an input/output (I/O) offset cancellation operation or a ZQ calibration operation.